Membrane depolarization and intracellular Ca2+ increase caused by high external Ca2+ in a rat calcitonin-secreting cell line

Down the membrane hole: Ion channels in protozoan parasites

Parasitic diseases caused by protozoans are highly prevalent around the world, disproportionally affecting developing countries, where coinfection with other microorganisms is common. Control and treatment of parasitic infections are constrained by the lack of specific and effective drugs, plus the rapid emergence of resistance. Ion channels are main drug targets for numerous diseases, but their potential against protozoan parasites is still untapped. Ion channels are membrane proteins expressed in all types of cells, allowing for the flow of ions between compartments, and regulating cellular functions such as membrane potential, excitability, volume, signaling, and death. Channels and transporters reside at the interface between parasites and their hosts, controlling nutrient uptake, viability, replication, and infectivity. To understand how ion channels control protozoan parasites fate and to evaluate their suitability for therapeutics, we must deepen our knowledge of their structure, function, and modulation. However, methodological approaches commonly used in mammalian cells have proven difficult to apply in protozoans. This review focuses on ion channels described in protozoan parasites of clinical relevance, mainly apicomplexans and trypanosomatids, highlighting proteins for which molecular and functional evidence has been correlated with their physiological functions.

The effect of maltobionic acid on bone metabolism markers in healthy Japanese postmenopausal women: A randomized double-blind placebo-controlled crossover study

Osteoporosis is characterized by compromised bone strengthpredisposing to an increased risk of fracture and is a disease with a high incidence in postmenopausal women. Frequent estrogen deficiency, particularly in postmenopausal women, induces osteoclast activation and is a major contributor to reduced bone mineral density. Maltobionic acid (MB) reportedly promotes mineral resorption and maintains bone mineral density in human clinical trials, although no studies have confirmed that MB improves bone metabolism in humans. Therefore, this study aimed to investigate the effects of MB administration on bone-resorption markers in healthy Japanese postmenopausal women. This was a randomized, double-blind, placebo-controlled, crossover trial. Twenty-six healthy adult Japanese women who realized that they had passed through more than 1 year of natural menopause and were aged 40-69 years were categorized into three groups. The experimental groups were allowed to consume maltobionic acid syrup 4 g (MB syrup 4 g group), maltobionic acid syrup 2 g plus maltose syrup 2 g (MB syrup 2 g group), and maltose syrup 4 g (placebo group) for 4 weeks. All 26 participants completed the intervention. Continuous ingestion of MB syrup 2 g or 4 g for 4 weeks significantly reduced the levels of bone-resorption markers deoxypyridinoline (DPD) and urinary N-telopeptide (u-NTx), and significantly increased the bone formation marker osteocalcin (OC) compared with the placebo group. Maltobionic acid (MB) intake may improve bone metabolism and reduce bone health problems, including osteoporosis, in postmenopausal, adult Japanese women. (UMIN-CTR ID: UMIN000038627).

New aspects of endocrine control of atrial fibrillation and possibilities for clinical translation

Hormones are potent endo-, para-, and autocrine endogenous regulators of the function of multiple organs, including the heart. Endocrine dysfunction promotes a number of cardiovascular diseases, including atrial fibrillation (AF). While the heart is a target for endocrine regulation, it is also an active endocrine organ itself, secreting a number of important bioactive hormones that convey significant endocrine effects, but also through para-/autocrine actions, actively participate in cardiac self-regulation. The hormones regulating heart-function work in concert to support myocardial performance. AF is a serious clinical problem associated with increased morbidity and mortality, mainly due to stroke and heart failure. Current therapies for AF remain inadequate. AF is characterized by altered atrial function and structure, including electrical and profibrotic remodelling in the atria and ventricles, which facilitates AF progression and hampers its treatment. Although features of this remodelling are well-established and its mechanisms are partly understood, important pathways pertinent to AF arrhythmogenesis are still unidentified. The discovery of these missing pathways has the potential to lead to therapeutic breakthroughs. Endocrine dysfunction is well-recognized to lead to AF. In this review, we discuss endocrine and cardiocrine signalling systems that directly, or as a consequence of an underlying cardiac pathology, contribute to AF pathogenesis. More specifically, we consider the roles of products from the hypothalamic-pituitary axis, the adrenal glands, adipose tissue, the renin–angiotensin system, atrial cardiomyocytes, and the thyroid gland in controlling atrial electrical and structural properties. The influence of endocrine/paracrine dysfunction on AF risk and mechanisms is evaluated and discussed. We focus on the most recent findings and reflect on the potential of translating them into clinical application.

Agonists and allosteric modulators of the calcium-sensing receptor and their therapeutic applications

The calcium-sensing receptor (CaR) belongs to the G protein-coupled receptor superfamily, with a characteristic structure consisting of seven transmembrane helices, an intracellular C-terminal and an extracellular N terminal domain. The primary physiological function of the CaR is the maintenance of constant blood Ca2+ levels, as a result of its ability to sense very small changes in extracellular Ca2+ (Ca2+(o)). Nevertheless, in addition to being expressed in tissues involved in Ca2+(o) homeostasis, the CaR is also expressed in tissues not involved in mineral homeostasis, suggestive of additional physiological functions. Numerous agonists and modulators of the CaR are now known in addition to Ca2+(o), including various divalent and trivalent cations, aromatic l-amino acids, polyamines, and aminoglycoside antibiotics. The signaling of the CaR is also regulated by extracellular pH and ionic strength. The activated CaR couples mainly to the phospholipase Cbeta and extracellular signal-regulated kinase 1/2 signaling pathways, and it decreases intracellular cAMP levels, leading to various physiological effects. The recent identification of synthetic allosteric modulators of the CaR has opened up a new field of research possibilities. Calcimimetics and calcilytics, which increase and decrease agonist signaling via the CaR, respectively, may facilitate the manipulation of the CaR and thus aid in further investigations of its precise signaling. These allosteric modulators, as well as strontium, have been demonstrated to have therapeutic potential for the treatment of disorders involving the CaR. This review discusses the various agonists and modulators of the CaR, differences in their binding and signaling, and their roles as therapeutics in various diseases.

Recent advances in physiological calcium homeostasis

A constant extracellular Ca2+ concentration is required for numerous physiological functions at tissue and cellular levels. This suggests that minor changes in Ca2+ will be corrected by appropriate homeostatic systems. The system regulating Ca2+ homeostasis involves several organs and hormones. The former are mainly the kidneys, skeleton, intestine and the parathyroid glands. The latter comprise, amongst others, the parathyroid hormone, vitamin D and calcitonin. Progress has recently been made in the identification and characterisation of Ca2+ transport proteins CaT1 and ECaC and this has provided new insights into the molecular mechanisms of Ca2+ transport in cells. The G-protein coupled calcium-sensing receptor, responsible for the exquisite ability of the parathyroid gland to respond to small changes in serum Ca2+ concentration was discovered about a decade ago. Research has focussed on the molecular mechanisms determining the serum levels of 1,25(OH)2D3, and on the transcriptional activity of the vitamin D receptor. The aim of recent work has been to elucidate the mechanisms and the intracellular signalling pathways by which parathyroid hormone, vitamin D and calcitonin affect Ca2+ homeostasis. This article summarises recent advances in the understanding and the molecular basis of physiological Ca2+ homeostasis.

Pixel-based criteria-oriented analysis of time-lapse Ca2+-fluorescence images

Since its inception, the analysis of time-lapse video-images acquired during Ca2+ imaging experiments using fluorescence microscopy has been progressively optimized for achieving a high temporal resolution. In contrast, the spatial resolution of the acquired images is often compromised during analysis to varying degrees by the need to draw regions of interest (ROI). We developed a strategy to analyze images at the acquired spatial resolution-pixel-by-pixel, grouping all pixels based on criteria of interest (COI) in regard to their associated fluorescence values over time and visualizing the distributions of the pixel-groups detected in a pseudo-colored map. We applied this pixel-based COI-strategy to the analysis of relative intracellular free calcium levels (Ca(i)(2+)) in attached cultured embryonic hippocampal cells under baseline and experimental conditions designed to evaluate the contribution of extracellular Ca2+ (Ca(e)(2+)) to baseline Ca(i)(2+) levels. We discovered distinct groups of Ca(e)(2+)-dependent Ca(i)(2+) regulation patterns emergent during the earliest phases of hippocampal cell differentiation, which were not limited to inter-cell differences. Thus, pixel-based COI-analysis of time-lapse images can be used to disclose distinct patterns of Ca(e)(2+)-dependent Ca(i)(2+) levels and their corresponding subcellular distributions in developing hippocampal cells. Such a strategy should be useful in studying the emergence and distribution of Ca(i)(2+) signaling at subcellular levels of resolution using fluorescence microscopy.

Proadrenomedullin N-terminal 20 peptide hyperpolarizes the membrane by activating an inwardly rectifying K+ current in differentiated PC12 cells

The mechanism of proadrenomedullin N-terminal 20 peptide (PAMP)-induced inhibition of catecholamine release from adrenergic nerve was investigated in nerve growth factor-treated PC12 cells that have differentiated characteristics somewhat similar to noradrenergic neurons. The effect of PAMP on the excitability of these cells was investigated with the use of perforated whole-cell clamp. PAMP hyperpolarized the membrane by increasing a K+ conductance in a dose-dependent manner. The current-voltage relationship (I-V) relationship of the PAMP-induced K+ conductance exhibited inward-going rectification. The activation was abolished by microinjecting GDPbetaS into the cells or pretreating the cells with pertussis toxin. These results indicate that a pertussis toxin-sensitive G protein is involved in the signal transduction. The PAMP-induced activation of the K+ conductance was attenuated by microinjecting antibody against the carboxyl terminus of Galphai3, but it was not influenced by microinjecting antibody against the common carboxyl termini of Galphai1 and Galphai2, which indicated that the G protein coupling the PAMP receptor to the inwardly rectifying K+ current is Galphai3. The PAMP-induced hyperpolarization may inhibit the catecholamine release from the neurons by attenuating the action potential frequency.

Effect of Ca2+ and cAMP on capacitance-measured hormone secretion in human GH-secreting adenoma cells

Membrane capacitance (Cm) was measured as an index of exocytosis in human growth hormone-secreting adenoma cells using the perforated whole cell, patch-clamp technique; the effects of membrane depolarization, growth hormone-releasing hormone, and 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP) were examined. Cm was increased by membrane depolarization to potentials beyond the threshold necessary to open voltage-gated Ca2+ channels. These voltage-dependent changes in Cm varied as a function of both depolarization amplitude and duration and were blocked in the presence of the Ca2+ channel antagonist nitrendipine (10(-6) M). When membrane potential was clamped at the holding potential (-78 mV), voltage-gated Ca2+ channels were closed, and neither application of growth hormone-releasing hormone nor 8-BrcAMP affected Cm. However, when these agents were applied to depolarized cells, where the voltage-gated Ca2+ channels were open, the increases in Cm were augmented. From these data, it was concluded that elevation of intracellular cAMP, per se, did not stimulate exocytosis. Rather, Ca2+ influx through voltage-gated channels was a prerequisite for cAMP-induced exocytosis.

Mechanism of extracellular Ca2+ receptor-stimulated hormone release from sheep thyroid parafollicular cells

1. Expression of receptors to extracellular calcium enables parafollicular cells of the thyroid gland (PF cells) to release calcitonin (CT) and serotonin (5-HT) in response to increased external Ca2+. Recently, a calcium-sensing receptor (CaR), similar to the G protein-coupled receptor for external Ca2+ cloned from parathyroid gland, was shown to be expressed in PF cells. Using a highly purified preparation of sheep PF cells, we have examined the electrical and biochemical processes coupling CaR activation to hormone release. 2. Whole-cell recordings in the permeabilized-patch configuration show that elevated extracellular Ca2+ concentration ([Ca2+]0) depolarizes these cells and induces oscillations in membrane potential. In voltage clamp, high [Ca2+]0 activates a cation conductance that underlies the depolarization. This conductance is cation selective, with a reversal potential near -25 mV indicating poor ion selectivity. 3. The CaR expressed in these cells is activated by other multivalent cations with a rank order potency of Gd3+ > Ba2+ > Ca2+ > > Mg2+. The insensitivity of these cells to high external Mg2+ contrasts with the reported sensitivity of the cloned CaR from parathyroid. 4. Elevation of [Ca2+]0 also stimulates increases in intracellular Ca2+ concentration ([Ca2+]i) and this effect is largely inhibited by the Ca2+ channel blocker nimodipine, indicating that L-type voltage-gated Ca2+ channels contribute to the response to elevated [Ca2+]0. 5. Elevated [Ca2+]0 induces an inward current under conditions where the only permeant external cation is Ca2+, indicating that influx via the cation conductance is another source of the increases in [Ca2+]i. 6. Extracellular Ca2+ stimulates 5-HT release with an EC50 of 1.5 mM. Nimodipine blocks 90% of the Ca2+0-induced 5-HT release, while other inhibitors of voltage-gated calcium channels had no effect. These data support an important role for L-type Ca2+ channels in CaR-induced hormone secretion. Although earlier studies indicate that high [Ca2+]0 induces release of Ca2+ from intracellular stores, thapsigargin-induced depletion of these stores did not affect secretion from these cells, indicating that Ca2+ influx is necessary and sufficient for the Ca2+0-induced 5-HT secretion. 7. Inhibition of protein kinase C (PKC) using chelerythrine, staurosporine, or calphostin C inhibited Ca2+0-induced 5-HT release by 50% while phorobol ester-induced 5-HT secretion was completely inhibited. Thus, PKC is an important component of the pathway linking CaR activation to hormone release. However, another as yet unknown second messenger also contributes to this pathway. 8. We tested the contribution of two different phospholipases to the CaR responses to determine the source of the PKC activator diacylglycerol (DAG). Selective inhibition of phosphatidylinositol-specific phospholipase C (PI-PLC) with U73122 had no effect on the response to elevated [Ca2+]0. However, pretreatment with D609, a selective inhibitor of phosphatidylcholine-specific phospholipase C (PC-PLC), inhibited Ca(2+)-induced 5-HT release to 50% of control indicating that phosphatidylcholine is a likely source of DAG in the response of PF cells to elevated [Ca2+]0.

Corticotropin-releasing hormone excites adrenocorticotropin-secreting human pituitary adenoma cells by activating a nonselective cation current

The mechanisms of corticotropin-releasing hormone (CRH) induced excitation of ACTH-secreting adenoma cells were investigated using the perforated whole-cell clamp technique and intracellular Ca2+ concentration ([Ca2+]i) measurement. CRH depolarized ACTH-secreting adenoma cells by activating a nonselective cation current that showed slight inward rectification. This channel did not seem to be a member of the Ca(2+)-activated cation currents because it was activated even when the [Ca2+]i was chelated below 50 nM. The activation of the current was induced by protein kinase A-mediated pathways. By [Ca2+]i measurement, CRH increased [Ca2+]i of these cells dependently on voltage-gated Ca2+ current. This CRH-induced [Ca2+]i increase was abolished in Na(+)-free extracellular solution, but was not abolished by the addition of 5 microM tetrodotoxin to the extracellular solution. CRH-induced ACTH secretion from the cultured adenoma cells was also abolished in Na(+)-free extracellular solution, but not in tetrodotoxin-containing extracellular solution. These data indicate that a Na+ current (maybe the nonselective cation current) other than voltage-gated Na+ current plays an important role in CRH-induced [Ca2+]i increase and ACTH secretion. CRH also activated a nonselective cation current in nonadenoma human corticotrophs, suggesting that the activation of a nonselective cation current is a physiological mechanism of CRH-induced excitation in human corticotrophs.

Activation of Cl- channels by extracellular Ca2+ in freshly isolated rabbit osteoclasts

Ionic channels regulated by extracellular Ca2+ concentration ([Ca2+]o) were examined in freshly isolated rabbit osteoclasts. K+ current was suppressed by intracellular and extracellular Cs+ ions. In this condition, high [Ca2+]o evoked an outwardly rectifying current with a reversal potential of about -25 mV. When the concentration of extracellular Cl ions was altered, the reversal potential of the outwardly rectifying current shifted as predicted by the Nernst equation. 4',4-diisothiocyanostilbene-2' 2-disulphonic acid (DIDS) inhibited the outwardly rectifying current. These results indicated that this current was carried through Cl- channels. Cd2+ or Ni2+ caused a transient activation of the Cl- current in contrast to the sustained activation elicited by Ca2+. Intracellular 20 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) inhibited the divalent cation-induced Cl- current. Either when the osmolarity of extracellular medium was increased, or when 100 microM cAMP was dissolved in the patch pipette solution, high [Ca2+]o still elicited the Cl- current, indicating that the divalent cation-induced Cl- current was carried through Ca(2+)-activated Cl- channels. Under perforated whole cell clamp extracellular divalent cations evoked the Cl- current, indicating that the activation of Cl- current did not arise from possible leakage of divalent cations from the extracellular medium under the whole cell clamp condition. This experiment further excluded a possible activation of volume-sensitive Cl- channels under whole cell clamp. Intracellular application of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) activated the Cl current and it was inhibited by intracellular 20 mM EGTA, suggesting that the activation of Cl current was mediated through a G protein, and that an increase in [Ca2+]i was critical for the activation of Cl-channels. A protein phosphatase inhibitor, okadaic acid (100 nM), caused an irreversible activation of the Cl current, suggesting that protein phosphatase 1 or 2A was involved in the regulation of Ca(2+)-activated Cl- channels.

Proadrenomedullin NH2-terminal 20 peptide inhibits the voltage-gated Ca2+ channel current through a pertussis toxin-sensitive G protein in rat pheochromocytoma-derived PC 12 cells

The effect of proadrenomedullin NH2-terminal 20 peptide (PAMP) on the voltage-gated Ca2+ channel current was investigated using the perforated whole-cell clamp technique on NGF-treated PC12 cells. PAMP inhibited the Ba2+ current through N-type Ca2+ channels in a concentration dependent manner. Injection of GDPbetaS into the cell abolished the inhibition while injection of GTPgammaS into the cell made the inhibition irreversible, indicating that the PAMP-induced inhibition of the voltage-gated Ca2+ channel was mediated by a G protein. The inhibition was abolished by pretreating the cells with pertussis toxin, indicating that a pertussis toxin-sensitive G protein was involved in the signal transduction mechanism of PAMP. The present study revealed that the inhibition of catecholamine secretion from sympathetic nerve endings by PAMP could be explained by the inhibition of N-type Ca2+ channels, which was mediated by pertussis toxin-sensitive G protein.

GHRH activates a nonselective cation current in human GH-secreting adenoma cells

Electrophysiological responses induced by human (h) growth hormone-releasing hormone (GHRH) were analyzed using the perforated whole cell clamp technique in human growth hormone (GH)-secreting adenoma cells. Application of hGHRH depolarized the membrane by increasing Na+ conductance. The reversal potential of the hGHRH-induced current was -20 to 0 mV. The channel was permeable to Na+, Li+ and K+ but not to TMA+. These properties were compatible with those of nonselective cation channels. Similar nonselective cation current was activated by 8-bromoadenosine 3',5'-cyclic monophosphate and forskolin, and the activation of the hGHRH-induced current was inhibited by protein kinase A (PKA) inhibitors, (R)-p-adenosine 3',5'-cyclic monophosphate and N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinoleinsulfonamide, and PKA inhibitor peptide PKI-(5-24), indicating that hGHRH-induced current was activated by PKA. Cholera toxin pretreatment eliminated the hGHRH-induced current, suggesting that Gs is involved in the activation of this current. This current became irreversible when the cells were pretreated with okadaic acid, suggesting that the recovery of the hGHRH-induced current was mediated by a serine/threonine protein phosphatase. GHRH-induced GH secretion was inhibited in Na+-free medium, suggesting the importance of the nonselective cation current on hGHRH-induced GH secretion. In human GH-secreting nonadenoma cells, hGHRH increased Na+ conductance, as was the case in GH-secreting adenoma cells.

A ryanodine receptor-like molecule expressed in the osteoclast plasma membrane functions in extracellular Ca2+ sensing

Ryanodine receptors (RyRs) reside in microsomal membranes where they gate Ca2+ release in response to changes in the cytosolic Ca2+ concentration. In the osteoclast, a divalent cation sensor, the Ca2+ receptor (CaR), located within the cell's plasma membrane, monitors changes in the extracellular Ca2+ concentration. Here we show that a RyR-like molecule is a functional component of this receptor. We have demonstrated that [3H] ryanodine specifically binds to freshly isolated rat osteoclasts. The binding was displaced by ryanodine itself, the CaR agonist Ni2+ and the RyR antagonist ruthenium red. The latter also inhibited cytosolic Ca2+ elevations induced by Ni2+. In contrast, the responses to Ni2+ were strongly potentiated by an antiserum Ab129 raised to an epitope located within the channel-forming domain of the type II RyR. The antiserum also stained the surface of intact, unfixed, trypan blue-negative osteoclasts. Serial confocal sections and immunogold scanning electron microscopy confirmed a plasma membrane localization of this staining. Antiserum Ab34 directed to a putatively intracellular RyR epitope expectedly did not stain live osteoclasts nor did it potentiate CaR activation. It did, however, stain fixed, permeabilized cells in a distinctive cytoplasmic pattern. We conclude that an RyR-like molecule resides within the osteoclast plasma membrane and plays in important role in extracellular Ca2+ sensing.

Inhibition of inwardly rectifying K+ current by external Ca2+ ions in freshly isolated rabbit osteoclasts

1. Regulation of membrane potential by extracellular Ca2+ concentration ([Ca2+]o) was examined in freshly isolated rabbit osteoclasts. 2. The resting membrane potential of osteoclasts was close to the K+ equilibrium potential in 1 mM Ca2+ medium. An elevation of [Ca2+]o caused membrane depolarization, accompanied by a decrease in the membrane conductance. 3. The inwardly rectifying K+ current observed under voltage clamp was dose-dependently inhibited by an elevation of [Ca2+]o, which explained the membrane depolarization caused by high [Ca2+]o. 4. Other divalent cations also inhibited the inwardly rectifying K+ current with the following order of potency: Ca2+ < Ni2+ < or = Co2+ < Cd2+. 5. In the presence of intracellular GTP gamma S the inwardly rectifying K+ current was irreversibly inhibited by [Ca2+]o, whereas the inhibition of the inwardly rectifying K+ current was greatly attenuated by intracellular application of GDP beta S. 6. Pertussis toxin (PTX) treatment did not abolish the inhibition of the inwardly rectifying K+ current caused by [Ca2+]o. 7. These results suggest that inwardly rectifying K+ channels in osteoclasts were regulated by a PTX-insensitive G-protein, which was coupled to the putative Ca2+ receptor or sensor on the cell membrane.

Interactions among calcium compartments in C6 rat glioma cells: involvement of potassium channels

1. Variations in intracellular free Ca2+ concentration ([Ca2+]i) induced by alteration of the extracellular concentrations of Ca2+ ([Ca2+]o) and K+ ([K+]o) were imaged in single fluo-3-loaded C6 glioma cells. In addition, the effect of membrane potential on [Ca2+]i was investigated in fura-2-loaded, voltage-clamped cells. 2. Step alterations of [Ca2+]o from 0 to 10 nM were followed by proportional variations in [Ca2+]i, with a maximum 7-fold increase and an apparent half-maximum at [Ca2+]o of 1.5 mM. 3. The time to half-maximum change (t1/2) of [Ca2+]o-associated [Ca2+]i variations ranged between 10 and 50 s, and was inversely related to the amplitude of [Ca2+]o steps. 4. Transient, serotonin-induced [Ca2+]i elevations, used as a measure of Ca2+ availability in inositol 1,4,5-trisphosphate-sensitive stores, were diminished within 10 min in 0 mM [Ca2+]o, but were unaffected by [Ca2+]o changes in the 1-5 mM range. 5. Restoration of normal [Ca2+]i following its elevation by serotonin was delayed by removal of external Na+ or Cl- and was enhanced by warming the medium to 37 degrees C. These conditions did not affect [Ca2+]o-associated [Ca2+]i variations. 6. [Ca2+]o-associated [Ca2+]i variations were depressed by La3+ and Ba2+, while blockers of voltage-activated Ca2+ channels were ineffective. 7. Elevated [K+]o depressed the basal level of [Ca2+]i, and in high concentrations (70-140 mM) also diminished the response to serotonin. 8. Depolarizing the membrane potential of voltage-clamped cells reversibly reduced [Ca2+]i. These membrane-potential associated [Ca2+]i variations were blocked by La3+, Ba2+ and TEA, all of which also depolarized membrane resting potential. 9. Apamin (at 1-10 microM), a blocker of [Ca2+]i-activated K+ channel, totally and reversibly prevented [Ca2+]o-associated [Ca2+]i variations. 10. These studies indicate that C6 cells are responsive to variations in [Ca2+]o, and that a K+ channel is a possible path through which Ca2+ penetrates into the cell.

Effects of activin A on ionic channels in human FSH-secreting tumour cells

1. Effects of activin A on ionic channels were examined in human FSH-secreting tumour cells using electrophysiological techniques. 2. Under voltage clamp with the conventional whole-cell clamp technique, the voltage-gated Na+ channel, the T- and L-type Ca2+ channels, the delayed K+ channel and the A-channel were observed. 3. With the nystatin-perforated whole-cell clamp technique, the same voltage-gated channels were recorded. Activin A (10(-7) M) increased the amplitude of the L-type Ca2+ current, whereas it decreased the amplitude of the delayed K+ current. 4. Under current clamp with the perforated whole-cell clamp technique, more than 80% of the cells exhibited spontaneous action potentials. Application of 10(-7) M activin A depolarized the membrane with a conductance increase and augmented action potential frequency. The reversal potential of the activin A-induced current was -20 to 0 mV. The activin A-induced current was abolished in a Na(+)-free extracellular solution, indicating that the membrane depolarization caused by activin A was due to the conductance increase to Na+ ions through non-selective cation channels.

Expression of Ca2+ receptors in Xenopus oocytes injected with poly(A)+ mRNA from a rat calcitonin-secreting cell line

Poly(A)+ mRNA extracted from rat calcitonin-secreting cells (rMTC 44-2) was injected into Xenopus oocytes. In mRNA-injected oocytes the intracellular Ca2+ concentration ([Ca2+]i), measured with the Ca2+ indicator dye, fura2, increased in response to an elevation of the extracellular Ca2+ ions ([Ca2+]o). In some oocytes [Ca2+]i transiently increased in high [Ca2+]o but it did not respond to the subsequent alterations of [Ca2+]o. The addition of 10 microM carbonyl cyanide m-chlorphenylhydrazone (CCCP) to the extracellular medium restored the dependence of [Ca2+]i on [Ca2+]o in such cells. It was concluded that rMTC 44-2 cells possessed a receptor which recognizes changes in [Ca2+]o and that these receptors can be functionally expressed by microinjection of messenger RNA from rMTC 44-2 cells into Xenopus oocytes.

Electrophysiological properties of rat calcitonin-secreting cells

The spontaneous electrical activity of calcitonin-secreting cells (C-cells) appears to play an important role in the coupling of fluctuations in the extracellular Ca2+ to changes in the intracellular Ca2+ concentration and thus for calcitonin secretion. Using the patch clamp technique, we have investigated the spontaneous electrical activity and the underlying ionic currents in C-cells of the rMTC 44-2 cell line. With 1.2 mM external Ca2+, the membrane potential was -46.1 +/- 1.7 mV (n = 58) and about 30% of the cells spontaneously fired action potentials. Rising the external Ca2+ to 1.8 mM caused the cells to depolarize to -42.1 +/- 2.1 mV (n = 56) and spontaneous electrical activity was seen in about 70% of cells. Under voltage clamp conditions, tetrodotoxin-sensitive voltage-dependent Na+ currents, outward-rectifying K+ currents and isradipine-, omega-conotoxin-sensitive as well as isradipine- and omega-conotoxin-insensitive Ca2+ currents were observed. These voltage-dependent currents appear to be the major ionic currents contributing to action potentials in C-cells and to participate in calcitonin secretion.